In an imaging system, an object of interest can be seen in focus and with negligible blur by changing the distance between lens and optical sensor accordingly. It is known that there are methods and systems to auto-focus an image. Such methods are used for example in infrared imaging systems (thermal cameras) in which there is a sensor acquiring an infrared image of the scene under consideration and generally these images are converted into a greyscale format. This image consists of a two dimensional array of pixels which represent the infrared intensity at these locations. Because of depth and range requirements of these systems, the depth of field is low and it is required to focus the image repetitively which lowers targeting and surveillance performance if done manually.
Currently, passive focusing is a widely used method, wherein the signal from the electronic image sensor is evaluated and used to drive the lens motion devices which are for example; stepper motors. One of the problems related to the passive methods is that, they require a mathematical function which represents the sharpness of the image. A good function should be monotonically increasing with a maximum at the focus point. There are many such currently used pixel-based functions. For example, the out-of-focus effect can be considered as a low-pass filter and the high frequency component of the image can be investigated to detect focus. A modified Laplace operator or a Tenengrad function is used, which both determine the high frequency component of an image by evaluating directional derivatives at each pixel location.
Another method uses local pixel variance of images which was observed to increase when the image is out-of-focus. Other than these Fourier transform method, total module difference method and histogram entropy method are other known methods to passively detect focus condition.
Another problem related to the automatic focusing systems is the processing power requirements due to the high frame rate and resolution of the infrared vision systems which makes image processing difficult on generic central processing units (CPU) or digital signal processors (DSP). Instead, field programmable gate arrays (FPGA) are used to process the data by parallel processing in much less clock cycles. Currently, many infrared vision systems are equipped both with FPGAs and DSPs which run histogram matching and edge enhancement algorithms on real-time. Current methods are not offering a simple way of focusing an infrared imaging system automatically although they can be used to auto-focus the lens with ease and very little modification using the method proposed.
The Japan patent document JP 3297282, an application in the state of the art, discloses a method for focusing an imaging system automatically by comparing derivatives of two evaluation value received from one low and one high pass filter and accordingly using one of these signals to determine lens position.
The International patent document WO 2010088079, an application in the state of the art, discloses a method for focusing an imaging system automatically using more than one digital band-pass filter which are switched according to local approximations of first and second derivative values.